Electromagnetic radiation generated in a device may disturb either the device""s own operation or the operation of some external device. Generally, the aim is to protect sensitive devices and interfering sources of electromagnetic radiation against radiation by encasing them in packages made from a conductive material and by sealing the packages so tight that no interfering electromagnetic radiation can penetrate the package. This type of or similar protection of devices against electromagnetic radiation is called EMI (electromagnetic interference) shielding.
One problematic area in EMI shielding is the sealing of junctions and joint surfaces comprised by devices, device cabinets and boxes. If the joint surfaces are not properly sealed with EMI gaskets, interfering electromagnetic radiation will quite easily pass through the joint. The best protection against interference is achieved when the joint surfaces are tightly sealed together galvanically. This means that resistance between the joint surfaces, so-called xe2x80x98junction resistancexe2x80x99, is as low as possible. However, it is difficult and expensive to manufacture such plane-like joint surfaces, where the surfaces are tightly attached to each other in every place galvanically. Therefore, solutions in which a good contact between the joint surfaces is not formed in every place but at certain distances along the whole length of the joint, are used for sealing joint surfaces. When the distance between the contacts formed is sufficiently short, electromagnetic radiation can no longer penetrate the joint in disturbing quantities. A sufficient contact distance depends on the frequency of the interfering radiation and the required attenuation level. Mechanical properties and the available space also affect the contact distance used. In connection with device cabinets and racks, a typical contact distance can be, e.g. 5-15 mm.
EMI sealing is required in various types of electric devices. Among others, EMI gaskets are used in device box and cabinet doors and apertures, as well as in partitions between different units inside device cabinets.
There are at least three types of gaskets that are most commonly used for EMI sealing. In one solution, a mantle is knitted from a conductive material around a resilient rubber compound or some other corresponding material. The mantle is knitted from a very thin wire that acts as a conductive fabric. When placed in between joint surfaces, these types of gaskets give an even contact but do not necessarily give a sufficient contact for EMI shielding due to the large contact area. They do not pierce through the surface, which is slightly oxidised or greasy. These types of gaskets may shed short pieces of wire, which can cause a short circuit after being passed on to a printed board. Neither do they endure friction and continuous wear.
In a second solution conductive particles are mixed inside a rubber-like sealing compound, the conductive particles forming a galvanic connection between joint surfaces when the joint surfaces are pressed together. However, the electroconductivity of these types of gaskets does not come near to that of, e.g. copper alloyed gaskets. Furthermore, the properties of these types of gaskets may change as they age.
A third solution is provided by spring-like gaskets bent from sheet metal. Their electroconductivity is good, but their manufacture is problematic. The manufacture of spring-like sheet metal gaskets requires expensive perforating and bending tools. In addition, the edges of the gaskets are sharp, whereupon one may hurt one""s hand on them, and the length of the gaskets is limited to the length of the sheet used in their manufacture, which normally is about 70 cm, in which case a full-length gasket must be assembled from several pieces.
The most significant disadvantage of a spring-like sheet metal gasket is, however, its susceptibility to being damaged due to its poor elastic properties. The gasket has extremely accurate tolerance of compression. If joint surfaces are pressed together too little, the gasket placed in between them will leak, as it is called, i.e. let electromagnetic radiation significantly through it. If again joint surfaces are pressed too much, a permanent deformation will take place in the gasket and its compression force will no longer be sufficient. Also in this case, the joint will begin to leak.
FIG. 1 illustrates an EMI gasket presented in the Patent Publication U.S. Pat. No. 5,091,606, which comprises a helical spring made from a circular profiled wire and may comprise a layer made on top of the spring from a conductive and ductile material. When this type of gasket is placed in between the surfaces to be sealed and the surfaces are pressed against each other, a contact is formed between the surfaces. The gasket is intended for sealing shafts and other surfaces with a circular cross-section, and their circumferences. A disadvantage of these types of gaskets is a reasonably complex manufacturing process, as well as the difficulty of fitting the gasket into small spaces.
Typically, the surfaces to be sealed are made from oxidable materials, such as sheet metal, in which case when the surfaces oxidise the contact between the surfaces becomes weaker and EMI tightness will be lost. The contact also becomes weaker when dirt gets between the gasket and the surfaces to be sealed.
Now, an EMI gasket has been invented with the help of which the disadvantages presented above can be mitigated. It is characteristic of an elongated EMI gasket made from an electroconductive wire that the cross-section of said wire comprises a shape protruding from a specific basic cross-section.
Correspondingly, it is characteristic of a device according to the invention, the device comprising a first part, a second part, joined together, and in between them an EMI gasket made from an electroconductive wire for getting said first and second parts into contact and for preventing electromagnetic interference from penetrating the joint part between the first and second parts, that the cross-section of said wire comprises a shape protruding from a specific basic cross-section for the EMI gasket to press against at least one of the surfaces of said first and second parts for forming an electric contact.
The properties of the sealing wire affect the contact forming properties of the gasket made from it. According to the invention, the EMI gasket is made from a spring-like electroconductive wire the cross-section of which comprises a shape protruding from a specific basic cross-section, e.g. an angle or a rounded angle, for forming a contact with a joint surface. An EMI gasket according to the invention is in direct contact with the joint surfaces and with it a sharper contact with the joint surfaces is achieved than with gaskets according to prior art, which do not comprise protrusions. When pressing the surfaces to be sealed together, the protruding shapes of the EMI gasket press effectively against the joint surfaces. This being the case, a permanent contact is achieved at short distances between the joint surfaces, and electromagnetic radiation cannot penetrate the joint in disturbing quantities.